Leg 175 Logging Summary
Shipboard Scientific Party
During Leg 175, thirteen sites were drilled off the western coast of Africa (figure 1) and six of them were logged. The objective of the Leg was to reconstruct the Late Neogene history of the Benguela Current and the associated upwelling regimes between 5°S and 32°S (figure 2). The area investigated contains one of the greatest upwelling regions of the world. The Angola-Benguela current system with its associated upwelling regions is characterized by organic-rich sediments that contain an outstanding record of productivity history.
Figure 1: Map showing the location of the sites drilled during Leg 175 and former DSDP sites.
Figure 2: Schematic representation of currents in the South Atlantic Ocean.
Most of the drilled sites have high sedimentation rates which offers an opportunity to develop detailed paleoceanographic records, with a resolution near 1000 yr. Physical properties of sediments continuously measured in situ with logging tools permit a detailed and continuous paleoceanographic and paleoclimatic study of the late Neogene.
Cored sediments are largely diatomaceous and carbonate-rich clays with variable (and occasionally very high) organic carbon contents. (See Berger, W., Wefer, G., Richter, C., et al., 1999)
The six holes listed in Table 1 were logged with a suite of sensors in order to continuously characterize the sedimentary changes for lithostratigraphy correlation and as proxy for paleoclimatic records, as well as to provide data for core-log integration. The logging tools were combined in four different tool strings as described in Table 1.
Table 1: Logged depth intervals during Leg 175
Main results from logging data
Natural gamma-ray data are well suited for paleoclimatic analysis. At Site 1077, the natural gamma log clearly follows the known galcial/interglacial stages of the delta18O isotope record. It shows that the glacial stages have high natural gamma radiation values. Thus, clay-rich intervals represent glacial sediments. A preliminary shipboard spectral analysis (Blackman-Tukey method) performed on the gamma-ray data shows the clear presence of both the eccentricity and obliquity orbital cycles as well as the precessional cycle (figure 3).
Figure 3: Spectral analysis calculated from the tuned natural gamma-ray data at Hole 1077A
At Sites 1081, 1082 and 1084, several dolomitic layers are well identified on log data. They are characterized by high velocity, resistivity, and density (figure 4). Because of their high resistivity, their position and thickness are clearly identified on the FMS images.
Figure 4: Density, resistivity and velocity at Site 1081. The peaks correspond to dolomitic layers.
At Sites 1084 and 1085, the presence of organic matter is visible in the downhole measurements by a general increase of the physical properties such as natural gamma-ray, density, resistivity, magnetic susceptibility and their correlation with a high uranium content. These organic rich levels are identified as black layers in the cores.
The post-cruise processing of the GHMT data recorded at Sites 1081, 1082 and 1084 have permitted to establish magnetic polarity sequences which are well correlated with shipboard paleomagnetic and biostratigraphic results obtained on cores.
Berger, W., Wefer, G., Richter, C., et al., 1999. Proc. ODP, Init. Repts., 175: College Station, TX (Ocean Drilling Program).
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